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BIKE AT A GLANCE

To adopt the right attitude for riding, it is necessary to understand the basic mechanism of a bike.

Let's Start with the Engine


The General Mechanism of an Engine:

There are two types of engines commonly used in Motorbikes the world over, namely: Two Stroke engines and Four Stroke engines.

Majority of engines used today are Four Stroke. The main working parts of the engine can be divided into two sections: The Reciprocating and The Rotary Sections.

The Reciprocating section of the Engine consists of parts like the piston, connecting rods, tappets, valves, etc, while the Rotary section consists of parts like the crankshaft, camshaft, ignition, timing gears etc…

To make an engine function, an air+fuel mixture containing vaporized fuel and oxygen (from the air) is supplied into the cylinder through the carburetor, where a spark ignites it so that it burns with an explosive force. The explosion, which takes place in a closed cylinder, tries to push the piston out of the combustion chamber, thereby causing the piston to move downwards. The reciprocating action of the piston is converted into a rotary action by the crankshaft, which uses the momentum of the piston coming down, to push the piston back up into the combustion chamber. This causes the piston to move up and down, in the engine. This rotary movement is then mechanically transmitted to the rear-wheel by means of a transmission system.

The carburetor is the heart of the engine, which regulates the petrol and air supply and prepares the air+fuel mixture for burning. It mixes approximately 15 parts of air with one part of petrol by weight and creates a fine spray of petrol by means of jets.

The air supplied from the carburetor passes through an air filter/cleaner which prevents dust particles from entering the engine.

The rider controls the amount of the air+fuel mixture entering the combustion chamber by operating the throttle. When the rider accelerates, a larger volume of the air-fuel mixture is fed into the combustion chamber. As a result of this, the intensity of the explosion taking place inside the closed cylinder increases. This causes the piston to be pushed out of the cylinder faster, which leads to an increase in the engine speed. The engine speed is then transmitted to the wheels which increase the bike's speed.


The Four Strokes:

Majority of internal combustion engines operate on the above mentioned 4 stroke cycle. The First Stroke or the movement of the piston from the top of the cylinder to the bottom is called the Induction (suction) stroke. The piston while moving towards the bottom of the cylinder creates a partial vacuum in the top of the cylinder which inducts/sucks-in/draws the air+fuel mixture through the carburetor (when the inlet valve is opened - explained later).

By the time piston reaches the bottom of the induction stroke, the cylinder is filled with the air+fuel mixture. When the piston moves up (and both valves are closed - explained later), the mixture is compressed or squeezed. This is known as the Compression stroke.

When the piston is about to reach the top of the cylinder, (known as TDC - Top Dead Center), the compressed mixture is ignited by a spark from the spark plug. This causes an explosion, which leads to the rapid expansion of the air+fuel mixture, which forces the piston to move downwards. This is known as the Power stroke.

The momentum of the piston caused by the power stroke causes the piston to rise up again in the cylinder which finally throws out the exhaust gases (since the exhaust valve is open - explained later) caused on account of the explosion of the mixture. This final movement of piston is called the Exhaust stroke. (The piston then once again moves into bottom of the cylinder to start with induction stroke) This is how Four Stroke Cycle operates.


The Role of the Valves:

Usually a cylinder has two valves. The valve, which allows the air+fuel mixture to enter the cylinder, is called the inlet valve and the valve that allows the exhaust gases to leave the cylinder is called the exhaust valve.

During the induction stroke the inlet valve remains open and the exhaust valve remains closed. During the compression stroke both valves remain closed. Both valves also remain closed during the power stroke. On the exhaust stroke, only exhaust valve opens allowing the burnt gases to be forced into exhaust system. This is how valves enable the sequence of the four stroke operation.

The opening and closing of valves and the sparking on the plug at the precise moment is obtained by the timing. The timing is governed by timing gears attached to the crankshaft, while the current to the spark plug is regulated through contact breaker points at the distributor.

The ignition of the air+fuel mixture inside the cylinder creates tremendous heat. Hence to prevent engine from overheating the engine has cooling as well as lubricating system. In most Indian built bikes, the forced air-cooling system is used, where in the air passes through the fins of the engine, which help in dissipation/dispersal of heat. The lubricating system, on other hand consists of oil which is kept inside the engine block, from where it is pumped around the engine to lubricate (almost 160) moving parts of the engine, Temperature over 700 degrees is produced inside the engine and only part of this heat is utilized to convert it into power. The remaining heat is thrown out through exhaust gases.

In order to maintain the engine in good running condition, it needs to be regularly tuned-up. This is done by cleaning and adjusting parts like the carburetor, spark plug(s), contact breaker point(s), air-cleaner/filter, oil-filter, tappets etc… The cooling and lubricating systems also need equal attention to avoid major pre-matured repairs to the engine.


The Transmission:

The engine converts the explosive energy to mechanical energy, through the reciprocating motion to rotary motion. The power developed from rotary motion is controlled by various systems in a bike. This system is known as the Transmission.

The Transmission consists of the clutch, gear-box & final-drive chain all the way upto the to driving wheel.

The clutch is a very important "Link" in the transmission of the bike. Its primary use is to allow the rider to engage and disengage the engine from the wheels. It also takes up most of the load and vibrations from the engine and does not allow it to pass onto the rest of the transmission.

After the clutch comes the gear-box. As the name suggests, a box having set of gears. It allows the rider to use the optimum amount of power from the engine as and how required. The main function of the gear-box is to reduce strain on the engine by supplying efficient power at required time. For Instance, the first gear is lowest and most sensitive gear, which helps in moving of a stationary bike without putting excessive load on the engine. On the other hand use of second gear requires relatively more acceleration and gives more strain on the engine and clutch. A lower gear serves two purposes. They serve as brake and also improve acceleration when required. For instance, while going downhill on a lower gear provides "Braking Power" and during uphill provides extra power.

The power transmission is by means of two chains. The primary chain is enclosed in the clutch case on the L.H.S. of the bike and runs in an oil-bath. The final or drive chain connects the gear-box to the drive-sprocket of the rear wheel. The drive-sprocket is coupled with the brake drum, which in turn is fit into the rear hub. There are 4 rubber blocks fitted between the drive-sprocket/brake-drum and the hub of the rear wheel, which act as efficient dampers, absorbing practically all the shocks of power and transmission units and have a very favorable influence on the life of chain as well as vital engine parts.


The Electricals:

The main components of an electrical system in the bike are the Battery, Alternator/Dynamo/ Generator/Magneto and Ignition/High-Tension Coil(s). The main function of the system is to provide ignition in the form of a spark to the compressed air+fuel mixture in the cylinder. A battery provides either 6 volts or 12 volts current. But a voltage thousands times higher is needed to create a spark from the spark plug, which can ignite the air+fuel mixture. It's the high tension coil (Ignition coil) which boosts low voltage current of the battery and provides upto 30,000 volts to the spark plug. A small spark is generated at the distributor, which is converted to a high voltage current, which flows to the spark plug and finally ignites the air+fuel mixture. A contact breaker and rotor, inside the distributor, ensure correct sequence of current to the plug while a condenser attached to the contact breaker serves as a capacitor that minimizes the damage to the contact breaker.

Another part, which is at the heart of the electrical system is the Alternator/Dynamo/Generator/ Magneto. This provides a charge to the battery by generating a current. As the generator output increases with the engine speed a control unit (Cut-out/Regulator) is provided to regulate the output. This unit prevents the damage to the generator unit and protects the battery from over charging/discharging. The stored energy of battery is used for different purposes such as: ignition, horn, lights etc….

To retain the standard energy of the battery it needs periodical maintenance like check up of specific gravity, cleaning of terminals, applying a thin film of petroleum jelly or pure Vaseline (not grease) to keep terminals and connections from corrosion and sulphation etc…


Brakes:

There are two different types of brakes used in bikes, namely: Drum Brakes & Disk Brakes.

The Drum Brake has an aluminum/steel/iron drum to which the wheel is attached. The drum and wheel rotate together. The brake shoe plate is bolted on to the chassis and inside the drum lie the brake shoes, which have brake liners on them. The brake liners are either riveted or moulded onto the brake shoes.

When the brake pedal is depressed, a cam (placed between the brake shoes) rotates such that the brake shoes move towards the drum. When the brake shoes grip the rotating drum, the rotating wheel locks/stops.

The Disk Brake has a metal disk instead of a drum fitted to the wheel and the calipers are bolted on to the chassis/shock-absorber pipe. The calipers have pistons and brake pads. The disk brake has to be operated hydraulically. The brake fluid and the actuating cylinder (brake-oil sump) are fitted on the handle bar of the bike. When the brake lever is pressed a non return brake valve operates and sends the brake fluids to the caliper and the piston, the caliper operates thereby forcing the brake pads to move towards the disk and produce the friction which allows you to slow down or stop.

The usual arrangement on a good bike is front disk brake and rear drum brake.


Suspension System:

The Front suspension consists of a telescopic fork with hydraulic dampers. It is the straight slider type with two cylindrical coil springs. Steel cover tubes protect the suspension elements.

The rear suspension operates on a circular path. The pivoted rear swing arm is sprung by two cylindrical coil springs and fitted with hydraulic dampers. The suspension dampers are protected by chromium plated steel covers.


The Wheels and Tyres:

The Tyres have two functions. First, they are air-filled cushions that absorb most of the shocks caused by riding on bad roads. Therefore they reduce the effect of the shocks. Second, the tyres grip the road to provide good traction; Good traction enables the bike to accelerate, brake and make turns without skidding.

There are two types of tyres available, the inner tube type and the tubeless type.

In the inner tube type tyre, both the tube and tyre are mounted on the rim. The tube is like a hollow rubber doughnut. It is inflated with air after it is installed inside the tyre and the tyre is put on the wheel rim. The inflation causes the tyre to resist any change of shape.

The tubeless type tyre does not have an inner tube. Instead, the tubeless tire is directly mounted on the wheel rim so that the air is retained between the rim and the tyre.


HAPPY TRAILS
Amol



   
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